Depending on how closely they're following the comics Phoenix Saga also probably means Hellfire Club. I'm going to guess Emma Frost first appears as an antagonist, then takes on a chapter 2 Storm-like roll before becoming a class teacher at the beginning of chapter 3. The devs have also said they want to add "flings," one-off non-romances. So Emma could possibly fill that roll for chapter 2, or be retconned into it later if the flings are further down the development calendar.Ima throw some guess here since chapter 2 is steadily approaching.
Since chapter 2 is labeled as the Phoenix Saga, going off that... If I were to guess, I think Emma has a decent shot of making a debut, but like mid year. Since she played a role on the Phoenix resurrection in one of the animated show (Although, I know that the dev is building their own stories, with some small correlation from the shows and comics, correct me if I'm wrong. I'm just throwing a huge curveball here).
Kitty Pryde had been in the comics for a decade at this point, so I'm going to guess she's going to be the inexperienced freshman who just discovered their powers and you have to help her learn to control them roll.
My guess for third girl is going to be somebody out of left field no one expects. She'll also be one of the "friends" that was gossiping about Jean in chapter 1. Not sure which character they'd use for this one. Unless the curve ball is they aged down Emma for this version. "Befriending" Jean just to gossip about her would be on-brand for Emma. If you want both you're going to have to get them to reconcile. It would tie nicely into Dark Pheonix being the chapter two boss (just my guess at this point).